To a large extent, the quality of any electroplate produced upon a metal part will depend upon the nature of the underlying surface. Generally, it will be of utmost importance that any substrate that is to be electroplated with chromium, nickel, or the like, or provided with other types of decorative or protective coatings, be very smooth and substantially free from defects. Grinding and polishing equipment can be used for that purpose; however, only a limited degree of improvement can be achieved in that manner, and such operations tend to impregnate the surface with fine abrasive particles and other foreign matter, necessitating subsequent treatment by pickling or aggressive cleaning. Mass finishing equipment (e.g., vibratory machines, open and closed vented tumbling barrels, and the like) are widely used to improve the quality of metal surfaces, but conventional practices generally require unduly extended periods of time, and in some instances ultimate refinement is not feasible. Finally, chemical finishing techniques (such as etching or bright dipping) are also widely employed, but they characteristically remove excessive amounts of metal from the surface, which is undesirable for a number of evident reasons, particularly when it is necessary to maintain close tolerances in the part being treated.
It is common practice to facilitate the action of cutting tools by applying certain substances to the surface of the workpiece. For example, in U.S. Pat. No. 2,298,418, Roesner et al disclose that higher cutting speeds and greater cutting thicknesses can be achieved by use of chemical solutions, such as phosphates. As is well known to those skilled in the art, in such techniques it is the cutting tool, rather than the metal surface, that is conditioned by the chemical substance; the objective is of course to enable a maximum bite and cutting speed, to achieve a high rate of metal removal.
In U.S. Pat. No. 3,593,410, Taylor teaches a method for casting and finishing tools or dies, wherein male and female members are matched and fitted together to effect removal of protuberances. The die numbers are mounted in a vibrating machine, and are submerged in an active solution (e.g., of copper sulfate) to chemically alter the matching surfaces; abrasive grits or grains may be interposed.
Certain esterification reaction products of phosphoric acid are used by Chang et al, in accordance with U.S. Pat. No. 3,932,243, to micro-etch the surface of a metal article; treatment is carried out in a conventional barrel or vibratory finishing machine. Mass finishing equipment is also used in cooperation with a chemical accelerator solution by Semons et al, in U.S. Pat. No. 3,979,858, to shorten finishing time and provide a smooth uniform surface on castings. The chemical accelerator solution employed comprises a lower aliphatic acid, and is maintained in the pH range 1.1 to 1.9; abrasive media may be included in the chemical solution.
Illustrative of other U.S. patents relating to surface modification, generally by chemical means, of metal articles are the following: U.S. Pat. Nos. 2,663,928 to Wheeler; 2,739,822 to Ellis, Jr.; 3,061,494 to Snyder et al; 3,259,517 to Atwell; 3,291,667 to Young, Jr. et al; 3,373,113 to Achenbach; 3,635,826 (reissued as Re. 27,662) to Hamilton; 3,650,861 to Angell; 3,654,001 to Mann; 3,719,536 to Rheingold et al; 3,905,907 to Shiga; 4,086,176 to Ericson et al; and 4,380,490 to Aspnes et al. In an article entitled "Vibratory Finishing With Chemical Accelerators", Safranek et al teach vibratory finishing processes for salvaging defective zinc die castings, and/or for preparing castings for electroplating, wherein a bright-dipping solution of sodium bisulfate and sodium dichromate is employed. Ten-fold acceleration of the finishing time, and the attainment of high quality surfaces, are reported by the authors.
Despite the widespread activity directed to the provision of techniques for the improvement of metal surfaces, as exemplified above, a demand remains for a process by which high levels of surface refinement (i.e., leveling, burr removal, and radiusing or edge improvement) can be produced quickly and efficiently, and with a minimal amount of metal removal.
Thus, it is the broad object of the present invention to provide a novel process by which metal surfaces can be refined to a high degree of smoothness, in very brief periods of time and with a minimal amount of surface metal removal.
It is a more specific object of the invention to provide such a process wherein the time required for surface refinement is reduced by a factor of about 25 percent to as much as 80 percent, from traditional processes.
Even more particularly, it is an object of the invention to produce a metal surface having an arithmetic average (AA) roughness in the range of five to ten in a period of less than six, and preferably about two to four, hours on a workpiece having an initial roughness value that is in excess of 15 AA to as high as 70 AA.
Further objects of the invention are to provide a process by which the productivity of mass finishing equipment can be greatly increased, the cost of surface refinement can be reduced substantially, and the need for subsequent treatment, such as by pickling and aggressive cleaning, can be minimized or eliminated.